The present invention is directed to a process of and apparatus for fabricating processed wood material panels similar to the apparatus disclosed in U.S. patent application No. 120,355, filed Nov. 13, 1987, now U.S. Pat. No. 4,882,837 issued Feb. 17, 1989. Processed wood material panels, such as chip board, fiber board, OSB (oriented structural fiberboard) panels, MDF (medium density fiberboard) panels and the like are produced from wood particles bonded together by a binder, such as urea, melamine and phenolformaldehyde resins with the addition of a hardener for effecting the curing of the panel. Initially, the wood material, along with the binder, is spread on a support surface to form a fleece which is subsequently compacted and pressed to form the panel. The hardener is introduced into the bonded wood particles before the fleece undergoes the compacting and compressing steps.
Such wood material panels as mentioned above are formed of wood chips, wood fibers and the like, bonded together by a binder. Usually, the binder is a urea, melamine or phenolformaldehyde resin. The wood material particles containing the binder are spread to form a fleece and the fleece is compacted and pressed in a pressing device to form a compacted panel. To accelerate the curing or setting process, a hardener is added to the binder for the processed wood particles. The setting process can be accelerated by supplying heat within the press.
To shorten the pressing time and reduce the supply of heat, the processed wood particles are bonded with a binder free of hardener and subsequently are spread to form a fleece and then compressed to form the processed wood material panels as disclosed in the U.S. patent application mentioned above. During the compacting step, an acidic or basic hardener in the gaseous phase, or binary phase with a gaseous carrier agent is supplied in the press along the surface of the fleece or directly into the interior of the fleece. As a result, the gaseous hardener must diffuse from the surfaces of the fleece into its center or flow through the fleece from its interior in the direction of the fleece surfaces during the compacting step. Accordingly, the concentration of the hardener in the fleece varies sharply during the compacting step as considered along its thickness, so that the time for pressing the material must be extended until the hardener concentration is the same everywhere throughout the fleece. Variable hardener concentration leads to irregular curing of the binder so that the structural properties of the processed wood material panel, such as tensile strength, resistance to swelling and the like, are eventually harmed.
It has now been found that the process disclosed in the U.S. patent application No. 120,355, mentioned above, can be improved whereby the time to complete the pressing of the panel can be shortened while improving the structural properties of the panels.